Attenuators are often required in an optical fiber transmission path to reduce the strength of an incoming signal to a desired level. Generally, the required attenuation is induced at a patch panel or a patch board because it is at this point that attenuators can be most conveniently inserted between mated connectors. For example, the prior art biconic connector disclosed in U.S. Pat. No. 4,213,932 employs a cylindrical shaped attenuator insert to space apart terminal free ends of two different optical fibers, thereby creating a "gap-loss" type of attenuation of the signal sought to be reduced in strength. In this prior art type attenuator, the attenuation is "molded in" and affixed to a receiving socket that has to be screwed into and out of a bulk head or a build out barrel, diminishing the flexibility of the connector, i.e., making it difficult to quickly change the value of the attenuator.
The disclosed invention employs the use of a removable low-loss, low-cost, frustroconical shaped shim or sleeve of a predetermined wall thickness in combination with conventional prior art biconic type connectors, and can be used regardless whether the incoming line is a bulk head feed through, or a built out variety. Once inserted, the attenuating shim or sleeve can be easily and readily removed or replaced with another. Consequently, an incoming line may be easily converted to a low loss situation and then converted back or then again changed to another predetermined loss with ease and speed.